What Components Make Up the Junction Capacitance Cj of ESD Protection Diodes?

1： Theoretical basis: Junction capacitance (Cj) includes depletion layer capacitance (Cd) and diffusion capacitance (Cs)

Cj= Cd+Cs

When the ESD tube is reverse biased (without breakdown), Cs can be ignored, Cj≈Cd， Its size is proportional to the PN junction area A and inversely proportional to the depletion region width W

Cd=εA/W，εThe dielectric constant of semiconductors

2： What is depletion layer capacitance (Cd)

Formation basis: When PN junction is reverse biased, a depletion layer will be formed, and the charge in the space charge region will change with the applied voltage. This charge change characteristic is equivalent to capacitance.

Influencing factors: mainly related to PN junction area, semiconductor doping concentration, and reverse bias. The larger the reverse bias, the smaller the Cd is usually

Practical significance: Cd can affect the high-frequency response of ESD diodes and is an important parameter to consider when selecting high-speed circuits

3： What is diffusion capacitance (Cs)

Formation basis: Under forward bias, multiple carriers will be injected into each other's region and stored, and the number of these stored carriers varies with voltage, equivalent to capacitive characteristics

Influencing factors: mainly related to the magnitude of forward current, minority carrier lifetime, and temperature. The larger the forward current, the larger the Cs is usually.

Practical significance: Cs mainly affects the low-frequency to mid frequency response of diodes. In ESD protection scenarios, its impact is much smaller than that of depletion layer capacitance (Cd) under high-frequency applications

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